This invention relates to high-capacity optical transmission systems in which each optical pulse is multiplexed to carry multiple bits of information.
Existing telecommunications systems typically transmit optical signals over optical fibres with a pulse rate of 10 Gb/s and, for example, use wavelength division multiplexing to transmit eight channels simultaneously, thereby achieving a data rate of 80 Gb/s per fibre. Transmission of optical signals is possible over about 400 km, using appropriate optical amplifiers, before conversion to the electrical domain is required to effect further regeneration.
It is estimated that future requirements of long distance traffic will require a transmission capacity greater than 1 Terabit per second per fibre. This increase in performance cannot simply be accommodated in such systems by increasing the bit rate per channel because of the onset of non-linear effects such as self-phase modulation and because the associated electronic processing at increased serial data rates becomes prohibitively expensive. Similarly, increasing the number of channels per pulse presents difficulties. Currently each channel is provided by a separate laser source whose ouptut is filtered to a respective channel wavelength, the wavelengths being separated by guard bands to provide suitable tolerance to drifting due to environmental effects or ageing and, coupled with the limited bandwidth of laser amplifiers used for optical repeating systems, a limitation therefore exists on the number of channels which can be added to expand the data handling capacity of the system.
It is known from De Souza et al, Optics Letters vol. 20, no. 10, p.1166-8, to provide wavelength division multiplexing using a single broadband femtosecond source by slicing the spectral bandwidth into 16 channels which are modulated individually. De Souza proposes using a diffraction grating and collimating lens to disperse light from the source onto a modulator array chip and to recombine the component wavelengths into an output beam for transmission to a receiver. A disadvantage of this technique is the size of the diffraction grating and associated optics.
A similar arrangement is proposed by Knox et al in U.S. Pat. No. 5,526,155 with the additional proposal that the diffraction grating could be replaced by a suitable wavelength splitter such as a Dragone wavelength router as described in C. Dragone, xe2x80x9cAn Nxc3x97N Optical Multiplexer Using a Planar Arrangement of Two Star Couplersxe2x80x9d, IEEE Photonics Technology Letters, vol. 3, no. 9, pp812-815, September 1991. The Knox reference proposes that each channel is formed by many different longitudinal modes of the optical source, the output of a pulsed laser typically having a spectrum in which a series of closely spaced peaks, sometimes referred to as modes of the laser. It is proposed by Knox that 250 or more different channel signals may be generated in this manner to provide a transmission capacity of 25 Gbits/sec.
It is known from Shao et al, xe2x80x9cWDM Coding for High-Capacity Lightwave Systemsxe2x80x9d, Journal of Lightwave Technology, vol. 12, no. 1, January 1994, to provide error detection and correction coding in a wavelength division multiplexed optical system where n channels are provided by separate sources of respective wavelengths and parallel word transmission occurs such that n=k+r where k equals the number of data bits per word and r equals the number of parity bits per word. A Hamming coding scheme is utilised to define the calculation of parity bits and the data recovery process at the receiver.
It is an object of the present invention to provide an improved multiplexing technique to allow transmission capacity to be increased to a rate in the region of 1 Terabit per second per fibre or more using a pulse repetition rate of the order of 1 GHz.
It is a further object of the present invention to provide error correction capable of dealing with both failure of individual channels in a multiplexed system and dealing with burst errors affecting a number of successive pulses. It is a further object of the present invention to provide an improved wavelength division multiplexing method with a large number of channels and controlling the stability of individual channel outputs.
It is a further object of the present invention to provide an improved method of generating clock signals when detecting multiplexed signals.
It is a further object of the present invention to provide an improved method of multiplexing optical pulses using a broad spectrum source.
It is a further object of the present invention to provide a method of optical communication in which the receiving and detection of received pulses is tolerant to systematic drift in the modulation and transmission of the optical pulses.
It is a further object of the present invention to provide a method of optical communication using multiplexed optical pulses which is tolerant to the occurrence of error prone channels.
According to the present invention there is disclosed a method of optical communication comprising the steps of;
transmitting a train of optical pulses;
multiplexing each pulse to provide a plurality of channels; and
applying error correction coding to data carried by the channels using both interchannel coding and serial coding of individual channels.
Data transmission is thereby rendered more robust under a range of error conditions such as the onset of failure of one individual channel and also the occurrence of a burst of errors affecting a number of channels simultaneously.
Preferably the serial coding is enhanced by interleaving. BCH codes such as Hamming codes may conveniently be used.
According to a further aspect of the present invention there is disclosed a method of wavelength division multiplexing of optical signals for use in optical communications comprising the steps of;
generating optical signals by operation of a single laser source which is pulsed to have a spectral content comprising a series of spectral lines;
inputting the pulses to a dispersive device for spatially dispersing a set of the spectral lines for each pulse into respective output components, the dispersive device comprising an array of waveguides having a range of incrementally different lengths arranged in a phased array configuration;
monitoring the extent to which the frequency selective properties of the waveguide array are matched to the spectral lines; and
controlling, in dependence upon the result of monitoring, the operation of at least one of the dispersive device and the laser source to maintain substantial uniformity with respect to time of the output components of the optical pulses.
The monitoring step preferably comprises monitoring the output of the waveguide array for a selected one of the spectral lines and controlling the operation of the dispersive device by regulating the temperature of a temperature controlled environment within which the waveguide array is located.
According to a further aspect of the present invention there is disclosed a method of optical communication comprising the steps of receiving optical pulses which are multiplexed to define a plurality of channels;
detecting the pulses to obtain temporally dispersed channel signals for the respective channels; and
generating clock signals for the respective channel signals for use in subsequent signal processing;
and wherein the generating step comprises extracting first and second clock signals in respect of first and second channel signals, and determining clock signals for remaining channel signals by interpolation.
Conveniently the first and second channel signals are selected as the earliest and latest received of the channel signals respectively. The interpolation in a preferred embodiment is linear with respect to temporal dispersion.
According to a further aspect of the present invention there is disclosed a method of optical communication comprising the steps of;
transmitting a train of optical pulses; and
multiplexing each pulse to provide a plurality of channels;
the multiplexing step comprising;
modulating the spectrum of the pulse with a set of spectral modulations associated with respective channels such that a respective channel value for each channel is represented by an amount of corresponding spectral modulation, wherein each spectral modulation is defined by a respective characteristic of modulation as a function of frequency and wherein the characteristics are mutually orthogonal in frequency space.
Such spectral modulation (referred to below as Fourier modulation) may conveniently be in the form of sinusoidal modulations in frequency space which may then be detected by Mach-Zehnder filters at a receiver.
Such spectral modulation enables the entire frequency content of the pulse to be utilised for each channel.
According to a further aspect of the present invention there is disclosed a method of optical communication comprising the steps of receiving optical pulses which are multiplexed to define for each pulse a plurality of channels such that a measurable parameter defining a property of the pulse has a first set of distinct values corresponding to respective channels;
detecting each received pulse by means of a detector array comprising detectors which are responsive to a second set of respective values of the parameter and which detectors output respective detector signals;
wherein the second set is greater in number than the first set so that the number of detector signals is greater than the number of channels;
and analysing the detector signals to extract channel signals representative of channel values carried by the respective channels.
The need for precise alignment of the detector array with any systematic drift in the measurable parameter is thereby avoided, as for example in the case of multiplexing by wavelength division multiplexing where the measurable parameter is optical frequency and the detector array consists of detectors responsive to dispersed frequency components.
According to a further aspect of the present invention there is disclosed a method of optical communication comprising the steps of;
transmitting a train of optical pulses;
multiplexing each pulse to carry a set of channels;
receiving the optical pulses;
monitoring for each channel a respective error rate in the received pulses;
selecting on the basis of most favourable error rate performance a subset of the set of channels to carry a first data stream;
designating remaining channels on the basis of being relatively error prone as non-selected channels to carry a second data stream; and
wherein the first and second data streams comprise respective error detecting codes whereby the monitoring step determines the error rate performance therefrom.
Optimum performance from the system may thereby be obtained by using the channels for which the rate of error detection is minimum.
The present invention also discloses communications systems and apparatus forming components of such systems for use in the above methods.
Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompany drawings.